Differential Vertical Shortening in Timber-Concrete High-rise Structures

Abstract

Differential vertical shortening (DVS) is the relative vertical shortening between two adjacent vertical load bearing elements. DVS originates because axial loading conditions on- and the design of adjacent vertical load bearing elements are different. Due to these differences, the reaction of the adjacent vertical structural elements expressed as length change can be different.

The aim of this thesis is to research differential vertical shortening effects in high-rise buildings that use a concrete core as a lateral stability system and a timber pendulum structure as a gravitational system. The Tallwood House in Brock Commons is a perfect example of such a structural system where the engineers have considered the shortening effects which will be reviewed in this research. Furthermore, this research will look for measures in the design and implementation phase that have influence on the magnitude of differential vertical shortening deformations.

In the Tallwood House project the problem was only tackled on basis of the individual column shortening and the effect it would have on vertical mechanical systems. A clear understanding of the relative difference between core and column shortening (DVS) and the effect it can have on secondary building systems is lacking. The determination of criteria on which the DVS deformations are assessed in the Brock Commons building, are based on the deformation capacity of the vertical mechanical systems, which does not necessarily relate to the DVS deformations. A more methodological approach is necessary for the prediction of DVS deformations and the determination of criteria regarding DVS deformations.

The sub question have the aim to reach to a more general and methodological approach in predicting DVS deformations and determining criteria for DVS deformations.

“What are the criteria on which differential vertical shortening deformations in the building structure should be assessed in order to guarantee the functional performance of the building during its service life?”

“How can differential vertical shortening deformations be quantified for a timber-concrete high-rise structure?”

After answering the sub-questions that relate to the criteria on DVS deformations and the quantification of DVS deformations, a research can be conducted to the influence of some input variables for the design and construction process to answer the main question of this research:

“What are effective measures which can be applied in the design and construction phase to mitigate the differential vertical shortening during the service life of high-rise buildings that uses a timber gravitational load bearing system with stabilizing concrete cores?’’

Problem areas where DVS deformations can have a negative effect on secondary building structures are researched. A general approach for the determination of criteria for DVS deformations is elaborated, based on principles for vertical deformation criteria that are described by the Eurocode.

The response mechanism of axially loaded concrete load bearing elements and axially loaded timber load bearing elements on external influences is researched qualitative by a literature study. The CEB-fip 2010 strain model is used to predict deformations in concrete. For timber, the strength class system is used according to Eurocode 5 to determine the elastic strains. The creep and shrinkage strains in timber are determined by experimental results that are found in a literature study.

The influence of the building process on DVS deformations is researched by a literature study. Staged construction analysis provides the possibility to simulate the sequential loading in a building process and to distinguish the DVS-deformations that can affect secondary building elements (post-DVS deformations) from the total-DVS deformations.

A case study on the Tallwood House at Brock commons in Vancouver is conducted to provide a basic geometric model in which DVS deformations can be predicted in which all previous research in this thesis serves as input properties of the model. From this basic model, variables in the design and building process are altered to conduct a sensitivity analysis of these variables. The building height expressed in number of floors, column cross section and the building speed of finishings in a building are taken as the analysed variables.

Increasing the column cross section can be an effective measure to decrease post- and total-DVS deformations due to elastic and creep strains. The effectiveness decreases with the increase of the column cross section because shrinkage strains are not affected. The post-DVS deformations can be decreased by postponing the building process of secondary structural elements in the construction schedule. The effectiveness decreases with increasing building duration because DVS-deformations due to loads of the finishes itself are not affected. Total-DVS deformations are not affected. Compensation has effect on the total difference in vertical position between the floor supports. The vertical translational movement is not prevented. This means that post-DVS deformations cannot be influenced by compensation.